Neurophysiology Hearing PH5208 PDF

Neurophysiology PH5208 The Auditory System - Hearing Dr. Moira Jenkins Sound is a Pressure Wave/Compression and Expansion of Air • Recall 3 parts of ear • Sound produced by pressure waves in the air • outer ear: collects sounds to focus upon the middle ear • middle ear: amplifies the sound energy...

Neurophysiology PH5208 The Auditory System - Hearing Dr. Moira Jenkins Sound is a Pressure Wave/Compression and Expansion of Air • Recall 3 parts of ear • Sound produced by pressure waves in the air • outer ear: collects sounds to focus upon the middle ear • middle ear: amplifies the sound energy for conveyance into the inner ear, cochlea • inner ear: transduces the sound energy into nervous signals Rhoades & Bell Fig. 4.15 2 Main Properties of Vibration of Air • Amplitude- size of the vibration This contributes to the perception of loudness, measured in decibels Larger amplitude louder the perceived sound 2 Main Properties of Vibration of Air • Frequency- cycles per second, Hertz • This contributes to the perception of pitch More frequent higher the pitch Less frequent  low pitch Sound waves are transduced into bioelectric signals within the inner ear cochlea  a coiled and rigid fluid-filled canal divided into three chambers separated by two flexible membranes: Riessner’s Basilar’s the ionic composition of the fluid varies among the three chambers: • perilymph: within the scala vestibuli and scala tympani • endolymph: within the scala medi hair cells comprise the sensory receptors that transduce sound energy into a neurochemical signal • the Organ of Corti contains the sensory apparatus for the transduction of sound energy • hair cells are arranged along an epithelial lining above the basilar membrane … Kandel & Schwartz Fig. 30-2 The structure of the middle ear provides “impedance matching” to transfer a sufficient strength of sound energy into the inner ear the middle ear ossicles: • malleus & incus: function as levers • stapes: functions as a piston sound waves striking the tympanum causes this membrane to vibrate with the same frequencies and amplitudes as the sound waves … Kandel & Schwartz Fig. 30-3 • the lever and piston arrangement of the ossicles transfers the pressure energy from the tympanum to the oval window, without distorting the frequency while amplifying the energy by 20-25 dB through this structure …  the oval window is made to vibrate with the same frequencies and relative amplitudes as the sound waves focused upon the tympanic membrane Sound-induced vibrations of the oval window transmit the energy of the sound waves into the cochlea • Back-and-forth displacement of the oval window by the stapes • Alternating pattern of compression and rarefaction in the perilymph of the scala vestibuli Vibrational motions of the oval window are transmitted through the fluid to the basilar membrane:  compression displaces the basilar membrane downward  rarefaction causes the basilar membrane to rebound upward Kandel & Schwartz Fig. 30-3 • Downward displacement of the basilar membrane (compression) will cause the pressure to build in the scala tympani … this compressioninduced pressure is relieved by the round window bowing outward • Likewise, upward displacement (rarefaction) will lessen the pressure within the scala tympani … this draws the round window inward Hair Cells- Sensory Receptors of the Organ of Corti • Modified epithelial cells, NOT neurons • Stereocilia extend from the apical end shortest to tallest • Kinocilium on tall end • Apical ends are bathed in endolymph • Neurochemical signaling and release neurotransmitter at synapse with the sensory afferent neurons of the cochlear nerve Organ of Corti • Projects into scala media, endolymph • Hair cells arranged in rows along the basilar membrane • Includes the tectorial membrane, gelatinous shelf over the hair cells -16,000 hair cells -arranged with 3 outer rows, tectorial touches, direct contact -1 inner row, tectorial does not touch • There are shearing forces along the 2 membranes Movement of the Stereocilia • The waves in the perilymph cause the basilar membrane to move up and down • This causes the tectorial membrane to move up and down too • Movement of the tectorial membrane generates shearing forces against the stereocilia for the outer hair cells • Inner hair cells do not contact the tectorial membrane; stereocilia on these are instead displaced by a shearing-induced movement of the endolymph below the tectorial membrane • The magnitude of the shearing forces varies in direct proportion to the amplitude of the sound waves • The frequency of the shearing is in direct proportion to the frequency of the sound waves [K+]endolymph > [K+]hair cells > [K+]perilymph • This creates a gradient that drives K+ into the hair cells • The back and forth movement of the stereocilia causes mechanically gated K+ channels to open • Depolarizes the cell • Voltage gated Ca++ channels open, NT release • to “complete this circuit” voltage-gated K+ channels along the base of the hair cell provide conductance for K+ to flow out of the cell into the perilymph, thereby preventing long-term accumulation of K+ within the cell Kendall & Schwartz 30.13 Neuroscience Fig. 13.10 Neurotransmitter release from the hair cell varies in a cyclic fashion relative to the back-and-forth displacement of the stereocilia Cytosolic [Ca++] varies relative to Ca++ entry via voltage-gated Ca++ channels Vm -40 mV (depolarized)  gCa increases Vm -60 mV (hyperpolarized)  gCa decreases  neurotransmitter release from the hair cell then varies relative to these cyclical variations in [Ca++]: • displacement towards …  increased release • displacement away …  decreased release glutamate -neurotransmitter released from hair cells, providing an excitatory input to the afferent nerve Rhoades & Bell Fig. 4.19 Transmission of sound energy into the cochlea induces a traveling wave that propagates along the basilar membrane • • • • • • • • The wave travels from the oval window to the apex (helicotrema) Distance traveled depends on the frequency of the sound The basilar membrane has a resonance frequency along this path specific frequencies of sound energy will therefore generate “resonant” (amplified) motion of the membrane at specific locations Low frequencies travel farther Low frequencies are detected at the apex Mid frequencies in the middle High frequencies at the base • Tonotopic map- localizing specific frequencies to locations on the Basilar membrane Tonotopic Map a given frequency of sound energy will therefore trigger signaling from just one location along the length of the basilar membrane  i.e. tonotopic mapping each nerve fiber is therefore said to be “tuned” to a specific frequency, based upon where along the basilar membrane that fiber arose • individual auditory nerves therefore convey information about a small portion of the audible frequency spectrum Most sounds consist of a mixture of pitches (different frequencies); auditory reception therefore breaks sounds down into their individual frequency components for transmission into the brain, where these components must then be recombined into the original sound Kandel & Schwartz Fig. 30.12 Tonotopic Mapping • Cochlear Nerve • Cochlear Nuclei • The arrangement of neurons within the cochlear nuclei maintain the tonotopic mapping of the sound energy arising from the frequencydependent excitation of hair cells along the length of the cochlea Kendal & Schwartz 31.3 Auditory information is relayed through several nuclei before reaching the primary auditory cortex • parallel pathways project from the cochlear nuclei to the inferior colliculus – To superior olivary complex – Or to lateral lemniscus • ascending auditory information then branches from the inferior colliculus – one pathway ascends to the medial geniculate nucleus of the thalamus, and from there to the auditory cortex – a second pathway projects to the superior colliculus, where it is integrated with visual input to assist in localization of objects, and orientation of the head and eyes Kandel & Schwartz Fig. 31.2 Perception of Sound includes Pitch, Volume, and Location is Space Location in space (source of the sound from outside the body): derived from comparisons of subtle differences in the input arriving from the two ears • auditory fibers from the cochlear nuclei split in partial decussation to send tonotopically matched information from both ears to the right and left superior olivary nuclei, which is the first location where bilateral comparisons are made • slight delays in arrival of similar signals from the two sides, and slight differences in intensity, are computed to localize the direction from which sound arises Kandel & Schwartz Fig. 31-5 (the answer is NO !!??)